I'm thinking of an alien alien world that has a vast biosphere. Now the creatures of this planet have evolved to be hexapods though there are a few creatures that are tetrapods (only have four limbs). Now I though it would be more locical to assume that the tetrapods evolved from the hexapods, but I'm not sure if that could really happen. So, as the title says, can a tetrapod evolve from a hexapod?

Edit: On what conditions would a hexapod creature need to be "exposed to" in order to lose the extra pair of limbs? What would be the most likely reason as for why they became tetrapods and why would only a handfull of them evolve like this?

$\begingroup$Down vote for "does not show any research effort".$\endgroup$
– RonJohnAug 19 '18 at 17:45

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$\begingroup$The simple answer to this question is "yes", as shown by a variety of internet sources and the thought experiments outlined in the comments to this question. Are you trying to ask "what are the evolutionary difficulties involved in a hexapod - > tetrapod transition"? If so, I'd recommend that you edit your question to reflect more accurately what you need help with.$\endgroup$
– DubukayAug 19 '18 at 17:55

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$\begingroup$hexapods have already lost several sets of limbs their ancestors had upwards of 10 sets of limbs, also the mouthparts of modern insects evolved from limbs.$\endgroup$
– JohnAug 19 '18 at 19:00

3 Answers
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Snakes lost their limbs because a certain gene may have turned-off their growth. The limbs are still defined by the DNA but are somewhat deactivated. A similar thing could happen to two out of the six limbs. The Earth analog is the greta oto butterfly.

"Could?" Of course it could.
"On what conditions?" Most likely a mutation.

Why? Because if all creatures have 3 pairs of legs, then you're kinda stuck with it. If your planet's biosphere has been dominated by hexapods the entirety of It's existence, then all life would have been adapted to it. If It's efficient and it works, there is no need to change it.

Yes, what you are looking for has already happened (as @Christmas Snow and other mentioned, snakes are the brightest example). We humans have undergone a similar process when we lost the tail (you may want to look for vestigial organs, like the said tail-bone).

There are examples of creatures emerging from the sea, developing legs, only to get back in the sea and lose those legs (yea, I'm talking whales, dolphins and other sea-mammals).

Another example of limb loss (as pointed out by jamesqf) would be the flightless birds, like the ostrich, emu, and especially the kiwi: wiki. Though they still have vestigal wings, they are so small as to be practically invisible.

What you should be really asking yourself is why your creature have lost their pair of limbs. As is the case with whales, they lost the limbs because they were detrimental in the sea, or again, we humans have no much use for a tail once evolved the erect position.

So, your tetrapods should have a pretty good reason to have evolved four limbs when your enviroment clearly favours six.

$\begingroup$Another example would be the flightless birds, like the ostrich, emu, and especially the kiwi: en.wikipedia.org/wiki/Kiwi#Description Though they still have vestigal wings, they are so small as to be practically invisible.$\endgroup$
– jamesqfAug 20 '18 at 16:27